1. Field of the Invention
The present invention relates to a machine tool and, for example, relates to a machine tool such as a boring machine for feeding a main shaft to perform boring.
2. Related Art
A horizontal boring machine has been conventionally used as one of NC (Numerical Control) machine tools. The horizontal boring machine has a main shaft which can be fed in a horizontal direction, this main shaft is fed in the horizontal direction (W axis direction) while being revolved and boring process of a machine target object is performed using a tool attached to a front end of the main shaft.
The horizontal boring machine has a support portion (a sleeve) which receives rotation of a motor, and the main shaft which is provided to revolve in synchronization with revolution of the sleeve. The main shaft has a rear end held by a ball bearing, and is configured to be fed along a sliding key in the sleeve in the horizontal direction (W axis direction). By this means, the horizontal boring machine can feed the main shaft in the W axis direction while revolving the main shaft.
The main shaft of the horizontal boring machine is supported by the sleeve and the ball bearing at the rear end. However, the sleeve does not move in the W axis direction, and therefore the supported state of the main shaft significantly changes according to the feed amount of the main shaft (the length of the fed main shaft). When, for example, the feed amount of the main shaft is a little (the feeding length is short), the entire main shaft is sufficiently supported by the sleeve and the ball bearing. By contrast with this, when the feed amount of the main shaft (the feeding length is long), the fed portion of the main shaft is not supported by the sleeve. This change of the supported state changes the critical revolution speed of the main shaft. When the revolution speed of the main shaft exceeds the critical revolution speed, the main shaft vibrates, and an excessive load is applied to the ball bearing at the rear end of the main shaft. In this case, processing accuracy decreases and, moreover, the ball bearing is likely to be damaged.
Although a machine tool having a small number of allowable maximum revolution of the main shaft or a machine tool of a little feed amount of the main shaft does not cause the above problem, it is demanded in recent years to revolve the main shaft at a high speed and process the main shaft in a complicated manner, and therefore the above problem cannot be neglected.
It is therefore an object of the present invention to provide a machine tool which can be controlled such that the revolution speed of the main shaft does not exceed a critical revolution speed even when the feed amount of the main shaft is changed.